Abstract Ultrasound (US) neuromodulation has received increased attention in recent years due to its unique ability to non-invasively activate and inhibit neurons. However, the mechanisms of US neuromodulation are not fully understood, and little is known about the optimal parameters that elicit neuromodulation. In this proposal, we will test a recently proposed model of US neuromodulation at the cellular level using patch clamp methods on pyramidal and interneurons, which have differing characteristics that we hypothesize will cause them to respond differently to US. US pulse parameters will be chosen using a fractional factorial design that will enable us to assess which aspects of the US pulse are most important for eliciting US neuromodulation. We will then translate this work to mice while measuring electrophysiological outputs and blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI). These experiments will allow us to assess whether findings at the cellular level hold in the whole animal and also to test the effects of US neuromodulation in the somatosensory network using BOLD fMRI and electrophysiological readouts. We will characterize the acoustic beam within the skull during these experiments using hydrophones, simulations, and magnetic resonance (MR) methods of imaging US beams, such as MR acoustic radiation force imaging. This quantification is important in interpreting US neuromodulation experiments, particularly in small animals, because their skulls act as reverberation chambers at the frequencies commonly used for neuromodulation. These studies will determine important spatial characteristics and limitations of US neuromodulation when used in the brains of small animals, where increased neuron density and reverberations likely cause proportionally larger effects to occur than in larger animals. In our final aim, we will use an array-based US neuromodulation system that is currently being developed in our lab to evoke activation patterns, and investigate the fine, middle, and long-range circuits in monkeys. This system can generate mm-scale foci through the monkey skull, which will enable exploration of the well-studied somatosensory system that is homologous to that in humans. In these monkeys, we will assess the effects of US neuromodulation over the parameter space identified in the first two aims using electrophysiological readouts and BOLD fMRI to map the S1 subregions of the somatosensory cortex during stimulation and quantify the effect of US parameters on BOLD fMRI to inhibit or excite the skin tactile evoked response. At the completion of the proposed studies, we will have an improved understanding of the cellular interactions of US with neurons, quantitative assessments of electrophysiological and BOLD fMRI activity that occurs at the network level, and an improved understanding of the parameter space that elicits US neuromodulation.